Microencapsulated adhesive beads are generally understood to comprise a shell which surrounds or encapsulates a liquid or solid adhesive core. The shell is impervious to the core material and is sufficiently strong so as to prevent exposure of the core during normal handling of the beads. However, upon the application of heat, pressure, mechanical force, or the like, the shell fractures, ruptures, dissolves, or is absorbed by the core thereby exposing the core. Microencapsulation is discussed in Microcapsules and Microencapsulation Techniques, by M. H. Gutcho (published by Noyes Data Corporation, Park Ridge, N.J., 1976) and Microcapsule Processing and Technology, by A. Kondo, edited by J. W. Van Valkenburg, published by Marcel Dekker, Inc., New York, N.Y., 1979. Described are limited utilities for the shell materials such as core retention, detackification, or as a portion of the adhesive system.
Two commonly employed techniques to produce microencapsulated adhesive beads are coacervation and in situ polymerization. In coacervation, a continuous shell is formed when a water soluble polymer is condensed from an aqueous solution. The shell forms about a nucleus of material which becomes the core. Shells of this type based on gelatin and gum arabic are well known.
For example, U.S. Pat. No. 2,907,682 "Adhesive Tape Containing Pressure Rupturable Capsules," issued Oct. 6, 1959 to H. J. Eichel discloses an adhesive tape comprising a web having a coating of two types of pressure-rupturable capsules thereon. One type of capsule contains a liquid solvent; the other contains a substantially solid adhesive that is soluble in the solvent. When pressure is applied to the tape, the capsules rupture and the adhesive and solvent become mixed. The capsules include a hard shell formed by coacervation from gelatin and gum arabic. These beads are coated onto the substrate in dispersion form and dried.
U.S. Pat. No. 2,988,460, "Adhesive Tape," issued Jun. 13, 1961 to H. J. Eichel discloses an adhesive tape comprising a web coated with pressure-fracturable capsules. Each capsule includes a hard shell which surrounds an adhesive core. The capsules are formed by coacervation and are coated in dispersion. Upon the application of pressure at a temperature substantially above 100.degree. F. (37.8.degree. C.), the shells fracture and the adhesive cores become tacky and flow. U.S. Pat. No. 2,988,461, "Adhesive," issued Jun. 13, 1961 to H. J. Eichel is similar to the immediately preceding patent except that the application of pressure without heat is required to activate the adhesive. In this case as well the adhesive is coated in dispersion form.
Japanese Kokai Patent No. 63-273680, "Capsule Type Adhesive and Adhesion Method Using Capsule Type Adhesive," Published Nov. 10, 1988, discloses an oil with an adhesive dissolved therein. The oil is sealed in a gelatin capsule. Applying pressure to the capsule causes the same to break thereby releasing the oil/adhesive blend. There is no mention of the coating technique involved using the capsule adhesive.
Japanese Patent Publication No. 60-124679, "Pressure Sensitive Adhesive Sheet" published Jul. 3, 1985 discloses several adhesive microcapsules. For example, FIG. 2(d) contained in Japanese Patent Publication No. 60-124679 illustrates a pressure-sensitive adhesive core covered by a fine inorganic powder and then encapsulated by a polymer film which is obtained by coacervation. Pressure is applied to the microcapsule to expose the adhesive core. The beads are subsequently coated using a simple primer coat with spray coating of the adhesive dispersion or dust coating of a dry adhesive material.
"In Situ" polymerization is a second commonly employed technique for producing microencapsulated adhesive beads. A shell formed of a gaseous, liquid, water or oil soluble monomer or a low molecular weight polymer is polymerized on the surface of a core material to provide a polymer film which covers the entire surface of the core material. Shells based upon urea-formaldehyde are well known. A variety of materials including homopolymers, copolymers, graft copolymers and block copolymers may be used to form the shell. For example, British Patent Specification No. 989,264, "Microcapsules and Method of Producing Them," published Apr. 14, 1965, discloses microcapsules comprising discrete, distinct and continuous aminoplast shell walls upon water-immiscible inert solid or liquid fill particles. No coating techniques are described. In situ polymerization is also mentioned in Japanese Kokai Patent No. 2-102280, "Microencapsulated Pressure Sensitive Adhesive Agent," published Apr. 13, 1990, which discloses a pressure sensitive adhesive agent in a non-pressure sensitive adhesive shell which surrounds the agent. A dust coating technique is employed.
Adhesive beads are also discussed in other publications. For example, U.S. Pat. No. 4,091,162, "Adhesives," issued May 23, 1978 to Henderson et al. discloses a "core-shell" polymer particle comprising a soft, tacky polymeric core surrounded by a hard, non-tacky non-blocking polymeric shell. The polymer shells render the adhesive beads non-blocking (i.e., non-agglomerating) in a latex dispersion which then may be coated from the dispersion, from solution, or by hot melt. The beads are formed by polymerizing the core followed by polymerizing the shell about the core. A typical "core-shell" polymer particle is illustrated in FIG. 1 of the Henderson et.al. patent.
Japanese Kokai Patent No. 2-102280 discloses a similar technique for producing a structure which includes an adhesive core and a non-adhesive shell which involves polymerizing a core followed by polymerizing the shell about the core. The above-mentioned Japanese Patent Publication No. 60-124679 discloses three adhesive microcapsules other than the coacervate structure illustrated in drawing FIG. 2(d). FIG. 2(a) shows an adhesive microcapsule in which a frozen and ground pressure sensitive adhesive is mixed with a rosin-like or terpene-like resin to form a powder that reportedly flows well at room temperature. The adhesive microcapsule illustrated in drawing FIG. 2(b) apparently comprises the adhesive microcapsule of FIG. 2(a) further coated with an inorganic powder such as silica, bentonite, alumina or talc so as to enhance the flowability of the microcapsules. The adhesive microcapsule of FIG. 2(c) comprises an adhesive core coated with an inorganic powder only.
Young et al., U.S. Pat. Nos. 4,833,179 and its divisional 4,952,650, "Suspension Polymerization," issued May 23, 1989 and Aug. 28, 1990, respectively, disclose the production of non-agglomerating pressure sensitive adhesive beads by suspension polymerization. The beads include an inorganic coating of silica powder which surrounds an adhesive core. Application of the beads by hot melt coating is described.
The above-mentioned references that describe an adhesive core surrounded by a shell fail to disclose a shell which has the ability to be used in any way in the positioning of the pressure sensitive adhesive core.
Japanese Patent Publication No. 62-3192, "Powder Adhesive for Electrostatic Gravure Printing," published Jan. 23, 1987, discloses the ability of a shell material to be electrostatically charged for the purpose of gravure coating of powdered hot melt adhesives. Only non-pressure sensitive adhesives with limited size (5.mu. to 40.mu.), charge levels, and charging methods (corona discharge) are disclosed. These are claimed to be useful for electrostatic gravure printing methods only.
U.S. Pat. No. 4,427,481, "Magnetized Hot Melt Adhesive And Method of Preparing Same," issued Jan. 24, 1984 to Mulik et.al. discusses installing a permanently magnetized ferromagnetic substance into a hot melt adhesive thereby creating a dispersion. It is then formed into a strip material which in turn can be positioned prior to activation of the hot melt adhesive. Upon application of heat the material flows and the magnetized particles draw the adhesive into the joint to be sealed. The patent does not teach a detackified PSA bead containing a magnetically responsive material.
Adhesive beads, in general, have been applied to substrates by a number of means such as from dispersions, from solutions, via hot melt applications and by dusting. Hot melt applications can be particularly disadvantageous in that the application process may require a high temperature which can result in the degradation of the adhesive. Furthermore, methods of pattern coating such as gravure coating can be inconvenient due to the need to substitute a new roll for each pattern which can be time consuming as well as expensive.